The present invention relates broadly to internal combustion engines and, more particularly, to so-called “variable stroke” engines wherein a mechanical connecting arrangement between reciprocating pistons and an engine crankshaft varies the extent of piston movement during the overall operational cycle of the engine. Generally, such mechanisms have the purpose of increasing the efficiency of internal combustion engines by achieving an effectively larger mechanical crank arm during the expansion stroke and an effectively shorter mechanical crank arm during the intake stroke.
Conventional internal combustion engines operate according to a repeating sequence of movements typically referred to as an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. In this sense, “stroke” describes the reciprocating movements of a drive piston as it travels back and forth through a cylindrical combustion chamber in an engine housing or “block.” The term “cycle” may also sometimes be used interchangeably with the term “stroke.” Hence, an engine operating according to the above-described manner is commonly referred to as a four-cycle or four-stroke engine, indicating that to complete a full power cycle the piston must reciprocate four times in the cylinder. “Cycle” is also used to describe the complete power cycle of the engine. This usage of terminology is consistent within and well understood to those skilled in the art.
As indicated, various designs have been advanced to cause an engine piston to travel a longer or shorter distance during the intake, compression, expansion, or exhaust strokes or during any combination of them, and to modify the piston velocity in some portion of its travel. For example, the so-called top or bottom dead center position of the piston has been shifted up or down for every revolution or every two revolutions. All of these conditions are different versions of a variable stroke engine. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 describe an extended expansion stroke. A practical application of an extended expansion stroke is the Millenia model automobile manufactured by Mazda, which utilizes a so-called Miller-cycle engine of the type designed in 1947 by U.S. engineer Ralph Miller. Miller's engines have been used for some time in ships and stationary power plants. The engineering goal is to reduce the engine's compression ratio without interfering with the power generating expansion stroke. In the Miller-cycle engine, the piston rises one-fifth of its stroke before the air intake valve is closed. After combustion occurs at the top of the stroke, the expanding gases push the piston all the way down to the bottom of the stroke, so the expansion ratio is not affected.
During the first half of the twentieth century, it was generally accepted among persons skilled in the art of internal combustion engines that the combustion products inside an engine cylinder had to be removed as completely as possible during the exhaust stoke following each expansion stoke and preceding the suceeding intake stroke. Many different patents propose differing ways to obtain a larger exhaust stroke. See, e.g., Hulse U.S. Pat. No. 1,326,733; Svete U.S. Pat. No. 2,394,269; Cady U.S. Pat. No. 1,786,423; Tucker U.S. Pat. No. 1,964,096; and Austin U.S. Pat. No. 1,278,563. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 also refer to a larger exhaust and expansion stroke. However, due to emission regulations implemented in the latter part of the twentieth century, new engine designs have been advanced wherein a portion of exhaust gas is recirculated or retained in the combustion chambers as a means of reducing the atmospheric emission of NOx (oxides of nitrogen) caused by the oxidation of nitrogen in the combustion chamber. This is accomplished by allowing intake manifold vacuum to draw exhaust gas into the intake manifold through an EGR (exhaust gas recirculation) valve.
Others have used variable stroke designs to modify the engine compression ratio. A lot of work has been done, especially in Europe and Japan, to achieve a so-called variable compression ratio by means of an arrangement that varies the position of the piston relative to the head of the cylinder.
The compression ratio is the ratio between capacity of the cylinder and capacity of the combustion chamber; in other words, the air-fuel mixture that goes into the cylinder during the intake stroke is then compressed as many times as the compression ratio value. Generally, the higher the compression ratio, the higher the engine efficiency. Some limitations such as mixture pre-ignition, knocking, engine temperature, and even engine construction exist. Since the compression ratio is one of the main factors affecting the engine efficiency, it is desirable to optimize it for different operating conditions (speed rate, load, acceleration, etc.). Schechter U.S. Pat. No. 5,165,368 describes a representative example of such an engine
A variable piston stroke application has also been utilized to optimize the pressure acting on the piston. For this purpose, the piston speed is decreased, relative to the speed of a conventional piston, near the top dead center to maximize the combustion process and the resulting forces acting on the piston. Schaal et al U.S. Pat. No. 5,158,047, Williams U.S. Pat. No. 5,060,603, and McWhorter U.S. Pat. Nos. 3,686,972; 3,861,239; and 4,152,955 are representative of this concept.
More recently, U.S. Pat. No. 5,927,236, discloses an internal combustion engine design wherein a gear set arrangement is utilized to connect the crankshaft and the piston connecting rods of the engine via offset bearing surfaces to accomplish a variation of the length of piston stroke over a complete engine power cycle. In particular this design seeks to increase the piston stroke via an increased effective crank arm length during the expansion portion of the power cycle to increase the torque output, and to reduce the stroke and piston velocity during the intake or admission and exhaust portions of the cycle to increase volumetric efficiency, all thereby enhancing the thermoefficiency of the engine.